2 links:
Ohm's law: Ohm's Law Calculators
Sensitivity Calculations: Pro-Audio and lighting Calculators. Calculator page

First off, IMO, the only reason we talk about impedance is because of SPL @ LF. The days of amps costing a lot and doing low(er) power numbers is what seems to have been the culprit for making lower final load drivers; you could take advantage of the amps available by re-configuring the VC of the drivers to get a higher load and thus get more power. More power = higher SPL from the same driver. We know this. This, however, doesn't necessarily mean that lower impedance is always better... or required. It's just a way of getting more output from a driver.

There is no standard for rating a driver's sensitivity. You typically see only 2 options: @ 2.83v/1m or @ 1w/1m. If I wanted to start my own company I could easily publish sensitivity specs at whatever I choose; 1.567659v/2.25m, 15w/20m, etc, etc. The norm is the two, though. Some mfg's will publish both just to help the consumer. That's really all there is to it.

Sensitivity, not impedance, is what you want to pay attention to. And with some pretty simple calculations given the links above, you can easily figure out what you need. Let's say you're shooting for 95dB @ 1/2m (typical in-car proximity from listener to drivers). Grab your 8 ohm, 88dB @ 1w/1m driver and use the sensitivity link above. You'll find that you need ~ 1.2w to achieve 94.8dB. What happens if you use an 4ohm driver with the same rating? Same result in terms of power required. No difference. The sensitivity rating in 1w/1m you're given isn't based on voltage. However, the voltage does differ between the two loads: 2.19v for 4 ohm, and 3.1v for 8 ohm.
If your driver is rated 88dB @ 1w/1m then it doesn't matter what the impedance is. You simply want to know what power (or voltage) is required of your amplifier to achieve XdB @ X distance. Simple as that.

Now, let's say the driver is instead rated in voltage.... but, rather than make an assumption that it's going to be 88dB @ 2.83v/1m, let's do some math to see what it most likely would be:
What voltage is needed at 1 meter to achieve 1w @ (say) 8 ohm load? Ohm's law: 2.83v.... hmmmm.... interesting.....
What voltage is now needed at 1 meter to achieve 1w @ 4 ohm load? Ohm's law, again: 2v.
Good to know. What you'll find - and shown above - is that a rating for 1w/1m of an 8 ohm driver is the same sensitivity at 2.83v/1m. You now also know that there is indeed a difference in a driver rated for voltage at varying loads.

To get a one-to-one for 1w/1m at varying loads, back track ohm's law using 2.83v @ 4 ohm and you'll get ~2w. That's twice the power of the 8 ohm.
Now, we can carry on to determining what power levels is required for each driver to achieve 95dB @ 1m. Use the link above, punch in 88dB for 1w/1m, punch in 1w for power, 1m for distance and you get.... 88dB. You should get this. It makes sense because you're asking what the dB level is at 1w/1m for a driver listed as 88dB @ 1w/1m. Now, if you have 2w/1m you just punch in "2" for your power rating and you now get 91dB.
Do this enough times and you begin to realize that for halving the load, the voltage decreases, which increases the power which gives you higher output at the same distance. The key is that you're still taxing your amp a bit harder to provide the current.

How much does this all matter, really, though? That's up to you to decide. Does your amp provide you with enough output, be it power or voltage, to get your speaker to the volume level you desire for a particular load?
The point of all this is to say: don't get hung up on impedance. If your amp has the ability to deliver the voltage (or power at X load) to meet the output needs for your system, you're good. Most amps are plenty capable of this. I wouldn't let the rating of a driver dissuade me from purchasing it without looking at what the SPL level is rated for. You may find that one driver you like in the 4 ohm variety has a lower sensitivity than the 8 ohm driver you like of another variety. Just make sure you're comparing apples to apples and in terms of output, dB is the name of the game. How you get there is totally up to you.

If I missed anything or screwed something up or you need clarification, shoot me an email. I don't have time to squat on the boards anymore like I used to.

- Erin